This invention relates to a technique for rotating the plane of polarization of linearly-polarized optical beams in the infrared portion of the spectrum; one application of the invention is the construction of an ultrafast switch for the control of infrared beams.
The inherent nature of a pulse of radiation, as opposed to a continuous wave, is that a pulse is localized or defined in time, while a continuous wave is not so defined. This time character renders pulses suited for a wide variety of applications: basic research in nonlinear optics; the use of time resolution spectroscopy as a probe for explaining physical laws; optical communication by digital data transmission; the use of the correlation between short pulses and high peak power to generate pulses with extremely high intensity; and the use of ultrashort, ultrapowerful infrared pulses as a tool for the achievement of controlled thermonuclear fusion.
In the range of visible light, a wide variety of pulse generation devices have been developed with picosecond (10.sup.-12 sec.) pulse-width lasers being commercially available. In the infrared, on the other hand, progress in the development of ultrashort pulses has been much slower. Current research efforts produce pulses that are on the nanosecond (10.sup.-9 sec.) time scale [see Abrams, Applied Physics Letters, Vol. 19, No. 518 (1971)] and are limited to that range of pulse widths by the use of electrical currents as a switching mechanism.
In the visible frequency range, apparatus described in a copending application by G. C. Bjorklund et al, Ser. No. 646,425 entitled "Polarization Rotator Based on Dispersion Due to Two-Photon Transitions", filed Jan. 5, 1976, achieves ultrafast switching by means of an interaction in a gaseous medium that rotates the plane of polarization of the incident light so that it is either transmitted or blocked by a polarizing filter. This rotation of polarization is effected in the prior art by taking advantage of the fact that a linearly polarized beam consists of two circularly polarized components with opposite sense of rotation and that a change of the phase of one of the circularly polarized components by some angle will rotate the plane of polarization of the linearly polarized beam by a related angle.
The mechanism by which the phase of one circularly polarized component but not that of the other is affected is the utilization of the angular momentum selection rules of two-photon atomic transitions. Units of angular momentum are combined according to a more complex rule than are simple numbers, and some combinations are forbidden by the principle of conservation of momentum.
When the atomic transition of interest is between two states of zero angular momentum, conservation requires that, when you start out with zero and end up with zero, you must have a net change of angular momentum of zero for the transition. Since a photon carries one unit of angular momentum, a transition between two states of zero angular momentum cannot be effected by the absorption of one photon. Such a transition can be effected by the absorption of two photons if they are combined in such a way that their angular momenta cancel and they are in a state of zero angular momentum.
The above-identified copending application by Bjorklund et al is based on the foregoing principles, making use of a zero to zero transition. The beam to be rotated will not be affected by the rotation medium when it is present alone because of the angular momentum limitations discussed above. A circularly polarized control beam is provided to effect the rotation. The control beam and the component of the incident beam of opposite circular polarization can combine in a zero state that will be affected by the zero to zero transition in the conversion medium, while the component of the incident beam with the same polarization as the control beam is not able to combine to give a zero net angular momentum and is therefore unaffected. When the sum of the frequencies of the incident and control beams is close, but not equal to the frequency associated with the transition, the phase of the interacting circularly polarized component of the incident beam will be affected, according to the Kramers-Koenig relation; but that component will not be absorbed, since significant absorption takes place only when the sum frequency is equal to the transition frequency.
Since the phase of one component has been changed by some angle, the combination of the two components will not result in the same plane of polarization as the original incident beam. Effectively, one half of the strength of the original beam has been delayed by some angle .phi., and the result will be a linearly polarized beam with the same intensity as the original, (since there has been dispersion but not absorption), with a plane of polarization that has been rotated by half the angle .phi..
When the angle of rotation of the polarization plane is 90.degree., the addition of a polarizing filter to the apparatus will form an optical switch. If the polarizer is set at an angle of 90.degree. from the incident plane of polarization, the incident beam will be blocked, except when the control beam's interaction rotates the incident polarization by 90.degree., so that it lines up with the direction of the filter and is transmitted. In this manner, the output from the apparatus is turned on and off by the control beam. The length of the output pulse is limited only by the length of the control pulse, which may be as short as a picosecond.
Although the apparatus disclosed in the above-identified copending application is a distinct improvement over the prior art in the visible range, it is not suitable for the infrared because of the limitations of the mechanism that was used in the rotation medium. The more populous levels of atomic vapors have available transitions that do not match well with the infrared. In a gaseous medium, also, the use of intermediate state resonances is limited by self-focusing and self-defocusing effects, (see copending application, Ser. No. 646,425, page 7, line 27; page 8, line 4). The avoidance of intermediate state resonance enhancement forfeits a great deal of the strength of rotation and requires the use of higher power in the control beam.